Series connected battery strings or batteries wired in series are used in a large number of applications and a large number of different vehicles, such as cars, boats, back-up systems, buses, trucks, golf carts, wheel chairs, electric cars and fork-lift trucks. Charging and discharging of such series connected batteries will inevitably result in a variance in voltage between different batteries in the string. If this difference is not corrected it will lead to an undercharging of some batteries and an overcharging of other during the charging of the batteries. This imbalance entails sulphating for lead-acid batteries (caused by undercharging) and drying up (caused by overcharging), which, in turn, will lead to that the charging level of the batteries will be below 100%, i.e. the batteries are not completely charged, and to a shortened duration of the batteries or even to battery damage. The charge process is also slowed down when the battery is reaching 100% state of charge due to apparent high voltage of the battery. The voltage difference which forces energy from the charge device to the battery is therefore reduced.
In order to avoid or prevent this voltage variance or imbalance between the batteries, a number of solutions have been proposed. A common approach is equalization, which is a technique that reduces the imbalances between the batteries aiming at equalizing the voltages of the different batteries of the string. Normally, an extended charging at a cyclic voltage or a low constant charging current is applied during an extended period of time at amplified voltages, thereby power from a battery with a higher voltage is shuffled to a battery with a lower voltage until they have an approximately equal voltage.
Another frequently utilized approach is to use a so called booster, which apply a voltage boost. This device increases the voltage to such a level that the charging is more efficient. It could however not handle the difference between different batteries in a string. Such a device is expensive if it is arranged to handle higher currents than approximately 8-12 A. In many applications, for example, buses, trucks, or fork-lift trucks current of approximately 100 A or more is common.
A third approach is to use a multi-stage generator in the engine. This type of generator could provide a controlled charge algorithm, but they are rather expensive. Furthermore, under certain conditions, it is preferred that the temperature at the battery is known in order to be able to apply a suitable charging current thus an additional temperature sensor must be located at the batteries and the temperature data must be transferred from the batteries to the generator. In many applications the temperature difference between the temperature at the batteries and the temperature at the generator can be forty degrees ° C. or more. Taken together this entails a complex construction and high costs as well as it may induce sensing errors.
Thus it is difficult to find a method and a device that provides a flexible, and reliable handling of the batteries of a battery string at a low cost and that can be used in a wide variety of applications, such as buses, trucks, golf carts, wheel chairs, electric cars and fork-lift trucks, etc.